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Evolution and Astrobiology
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Imagining Other Earths
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Extremophiles
Extremophiles are life forms that flourish in extreme environments—regions of very high (or low) temperatures, acidities, and even intense radioactivity. The lecture describes some of these fascinating life forms including the bacterial mats of Yellowstone and Deinococcus Radiodurans, the toughest creature on Earth. These extremophiles inform our understanding of astrobiology.
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David SpergelCharles Young Professor of Astronomy on the Class of 1897 Foundation and Chair
Department of Astrophysics
I've always thought of the Chernobyl reactor as one
of the more unpleasant places on the planet.
Not a place that I imagine life thriving.
Yet, we've gone and looked at what's at Chernobyl,
what's on the reactor walls.
The walls of the reactor are covered with black fungus.
In fact we also see this at the space station.
Another pretty hostile environment.
There are fungi that survive and thrive in environments filled with radioactivity.
These fungi not only have mechanisms for
surviving radioactivity, these fungi are what we called Radiophilic.
They like the radioactivity.
They make use of it as an energy source.
What these fungi do, is they make use of melanin, right?
This is the stuff that gives our skin color.
And the melanin absorbs the radioactivity, uses it for
chemistry, and the energy source for
things like this Cryptococcus Neoformans is radioactivity.
Scientists have done experiments,
where they've exposed this to 500 times the normal radioactive background, and
found that this stuff does better when exposed to radiation, then left alone.
It's very clearly making use of that energy.
These forms have very interesting adaptations,
that let them survive this highly radio active environment.
What radiation does to life, is it tends to destroy and chop up things like DNA.
X-rays or cosmic rays will come along, high energy particles or
high energy radiation interact with the DNA in our bodies, or
in the bodies of most lifeforms, and cause the DNA to be chopped, sites to change,
it'll actually change the sequence of our DNA, and change our biochemistry.
Most of the time, those changes do nothing.
They just take a functioning cell and make it non functional.
But every now and then, radioactivity will produce cancer.
Whereas it will cause a mutation that can be extremely harmful.
These radiophilic fungi get around this by having special adaptations.
And an example is Cladosporium Sphaerospermum.
It has multiple copies of every chromosome.
So it has a lot of redundancy.
So when one chromosome is destroyed and damaged, there're other copies, and
there are DNA repair mechanisms that let it undo the damage of this radioactivity.
Perhaps the most striking of this radiophillic lifeforms,
is Deinococcus Radiodurans.
This bacterium is arguably the toughest creature on the planet.
It seems capable of surviving almost everything.
It can survive extreme heat, extreme desiccation, you can dry it out for
a very long period of time, and very high doses of radioactivity.
It can survive doses of radioactivity that are 3,000 times
what would kill you and me.
It has a very interesting structure for its DNA.
Its DNA forms these tightly-packed toroids that make it stronger.
It's designed in a way in which when radiation comes along and
breaks strands in the DNA, there are repair mechanisms that
are part of its biochemistry, that lets the DNA repair.
Not only that, it makes use of the fact that there're multiple
copies of the DNA, to effectively have what on a computer
would be error correcting code that notices there's
a wrong version of the DNA and a mutation, and corrects for it.
Plus there're other copies that have not suffered that mutation.
How tough is Radiodurans?
Even tougher than Chuck Norris.
And if that internet doesn't mean anything to you,
after the lecture you should Google how tough is Chuck Norris?
And you'll get a lot of bad jokes.
Well, lets go back to Radiodurans.
Radiodurans is a remarkable survivor.
There was a very impressive experiment done by Radman and his collaborators,
where they took Radiodurans, they chopped it into lots of little fragments, and
they found a completely unknown mechanism that hadn't been seen before.
Now called extended synthesis-dependent strand annealing that brought it
all back together.
These strands cut them in many, many places.
Have these broken strands you know, effectively lying on the floor,
or lying in your gel and
they will reassemble themselves into functioning Radiodurans.
This truly is the Chuck Norris of bacteria.
We see that Radiodurans is tough.
It's interesting to ask why is it so tough?
Why did it evolve the mechanisms to let it
survive these environments of extreme radioactivity?
We don't normally have environments of extreme radioactivity on Earth.
You know, there are places on the planet where the radiation doses are higher than
others, because of the rocks, there're places in parts of Iran and India
that happen to have radiation doses that are even 100 times, above the normal dose.
But nothing, like the levels that Radiodurans is capable of surviving in.
Why did it evolve this capability?
And there's broadly two sets of answers that I've seen offered to this question.
One is it evolved this capability, because it
evolved to survive its conditions of extreme drying, of desiccation.
It survived, evolved to survive environments where
the Radiodurans would be in completely dried out.
That desiccation process would destroy the DNA, and it would then survive.
A more intriguing possibility, is that Radiodurans survived or
evolved, to survive space travel.
That the only place we know of where you have these very high radiation dosage,
is in space.
And perhaps, Radiodurans survived to be able to make the trip from Mars to Earth,
or perhaps to be able to travel even further in the galaxy.
We don't know where Radiodurans came from, but it has really intriguing properties.
It's very intriguing to see how all these extremophiles fit into our tree of life.
Recall when we talked about the tree of life in the last lecture.
What we do with the tree of life is we take the ribosome of
different creatures and we sequence it.
And we see how different bacteria and archaea and
eukaryotes are all related to each other.
And here we are.
We live up near the crown of the Eukaryatic part of the Tree of Life,
pretty far away from the common ancestor of all life.
But if we look at the Extremophiles and see where some of the Extremophiles live,
where do we find things like Aquifex?
And a number of these thermophilic lifeforms are ones that
thrive on methane, are some of the halophiles we've talked about.
These extremophiles, we see happen to live or
be located right near the base of the tree.
So it looks like either these extremophiles split off very
early from a non-extremophile ancestor, that then evolved into this branch here.
Or, early life started out as an Extremophile.
Life itself originated in one of
these very extreme environments that we've talked about in this lecture.
And then evolved from thriving in this extreme environment,
to be able to thrive in the more typical environments of modest temperatures,
comfortable pressures kind of ordinary PHs and salinities.
So it could be this is where we started with some of these extremophiles.
And this is where we end up here, today.
And I've mentioned earlier, these extremophiles are also
very interesting for thinking about astrobiology.
We don't know whether there's life beyond Earth.
We know that there're lots of planets out there,
but as I talked about in the previous lecture,
we don't know how hard it is to go from the basic building blocks of life,
the amino acids and the proteins, to form simple life that can evolve.
But, the fact that we have now found these extremophiles that
thrive in such a wide range of environments.
And many of these extremophiles are lifeforms that we've only learned about
in the past decade or two, has broadened our minds.
And we now realize that life can thrive in a very wide range of environments,
and perhaps one of those environments is all around, or around another planet.
Around another star somewhere in our galaxy.
I'll leave you with that thought.
Thanks.
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